Microfine cementitious compositions are generally similar to standard types of portland cement, except that such compositions have finer particle size distributions. These finer particle sizes, together with differences in chemistry, result in early setting, high strength cements. High early strength cement is desirable in numerous applications. For example, a microfine cementitious compound may be used to grout defects of oil and gas wells, that is, to fill the microscopic cracks, voids, and fissures which may be present in the formation or annular cement. Such microscopic defects are generally on the order of about 50 microns in size. Those defects are repaired when the microfine cement sets within the defects, resulting in the repair of the defects. Similarly, a microfine cementitious compound may be used to grout microscopic defects within bridges, dams, roads, airport runways, or other infrastructure projects.
Numerous physical properties determine the efficacy of microfine cementitious compositions in remediating microscopic defects. For example, a cement slurry formed from a microfine cementitious composition should be pumpable into the particular structure being remediated. Pumpability may be evaluated by reference to both the setting time and the viscosity of the cement slurry. If the slurry is too viscous, or sets too quickly, it will not be sufficiently pumpable. On the other hand, if the slurry is too viscous or sets too slowly, it may not set within the microscopic defects. The slurry should exhibit sufficiently high compressive strength to effectively reinforce the microscopic defects.
Microfine cementitious compositions make up a negligible portion of total cement produced when compared to the volume of overall portland cement production. There are five basic types of portland cement outlined by American Society for Testing and Materials (ASTM) Specification C150 and American Association of State Highway and Transportation Officials (AASHTO) Specification M 85. These standards specify such physical properties as chemical composition, fineness, soundness, consistency, setting time, compressive strength, heat of hydration, specific gravity, and loss on ignition (L.O.I.). Each one of these properties has an influence on the performance of the cement, and the different types of portland cement are therefore suited to particular applications.
For example, Type I cement is a general purpose cement used in the majority of construction, such that approximately 90% of all cement produced falls within this category. Type II cement may generally be used when a moderate sulfate resistance is desired. Type III cement may generally be used when a relatively high early strength is desired. Type III cement is similar in composition to Type I cement, except that it is much finer, with a typical Blaine fineness of about 5500 cm2/g compared to a typical fineness of about 3500 cm2/g for Type I cement. Type IV cement may generally be used when a low heat of hydration is desired, while Type V cement may generally be used where high sulfate resistance is desired. The typical chemical composition of these cement types, in % by weight, is approximately as follows:
TypeType IType IIIIIType IVType VC3S 55% 51% 57% 28% 38%C2S 19% 24% 19% 49% 43%C3A 10%  6% 10%  4%  4%C4AF  7% 11%  7% 12%  9%SO32.9%2.5%3.1%1.9%1.8%MgO2.8%2.9%3.0%1.8%1.9%L.O.I.1.0%0.8%0.9%0.8%0.9%
The setting of cement involves the chemical reaction of hydration. In general terms, the early hydration of cement is principally controlled by the amount and activity of C3A, balanced by the amount and type of SO3 interground with the cement. As the fineness of C3A within a cement increases, the activity of the Al2O3 rises due to an increase in reactive surface area. Consequently, the greater the fineness of C3A within a cement, the higher the concentration of SO3 required to inhibit the Al2O3. C3A hydrates very rapidly and will influence early bonding characteristics. If this hydration is not properly controlled by the sulfate, problems such as flash set, false set, slump loss, and cement-admixture incompatibility may result.
Of particular concern is the problem of false set, also known as early stiffening or premature stiffening, which may occur when an excessive amount of hemihydrate, i.e., the partially dehydrated form of gypsum (CaSO4.½H2O), is present in the cement. In particular, false set is primarily caused by the rehydration of hemihydrate back to gypsum, which precipitates out of the cement slurry. This gypsum precipitate causes the appearance of setting, when in reality the hydration of C3A has not yet begun in earnest, and is further inhibited by the presence of gypsum.